Traumatic Heterotopic Ossification Workup

Updated: Oct 30, 2019
  • Author: John B Wood, MBBS, FRCS(Edin), FRCS(Tr&Orth), FEBOT, Dip Sports Med (UNSW); Chief Editor: Harris Gellman, MD  more...
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Workup

Laboratory Studies

Alkaline phosphatase (ALP) levels can be used to evaluate heterotopic ossification (HO). If the serum ALP level is raised, inorganic phosphate should be assayed because this level should also be raised and accompanied by a transient decrease in serum calcium level. The ALP level may be as high as three to four times the normal level, peaking at about the 12-week stage. A prolonged increase in the ALP level can be of prognostic value because this may indicate ongoing osteoblastic activity.

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Plain Radiography

Plain radiographs are useful in the diagnosis of HO. The appearance of the hip is classified according to the Brooker grading system (see the images below), [8]  which makes use of the following four grades:

  • Grade I - Appearance of islands of bone within the tissues
  • Grade II - Spurs of bone emanate from either the femur or the pelvis, with gaps of more than 1 cm between these spurs
  • Grade III - Gaps between spurs are less than 1 cm
  • Grade IV - Apparent ankylosis of the hip caused by the HO
Brooker I heterotopic ossification associated with Brooker I heterotopic ossification associated with an uncemented total hip arthroplasty.
Brooker I heterotopic ossification associated with Brooker I heterotopic ossification associated with a cemented total hip replacement that has undergone acetabular component augmentation and fixation of the greater trochanter.
Brooker I heterotopic ossification associated with Brooker I heterotopic ossification associated with a revision hip arthroplasty.
Brooker I heterotopic ossification associated with Brooker I heterotopic ossification associated with bilateral revision hip arthroplasties.
Brooker II heterotopic ossification associated wit Brooker II heterotopic ossification associated with a right cemented total hip replacement. On the left side, an uncemented total hip arthroplasty is present with no heterotopic ossification.
Brooker II heterotopic ossification associated wit Brooker II heterotopic ossification associated with a right revision hip arthroplasty.
Brooker III heterotopic ossification associated wi Brooker III heterotopic ossification associated with a left uncemented total hip arthroplasty.

An alternative grading system was developed by Schmidt and Hackenbroch that is more complex than the Brooker system. [9]  This system classifies HO according to its location or region (by number) and extent (by letter), as follows:

  • Region I - HOs are strictly below the tip of the greater trochanter
  • Region II - HOs are below and above the tip of the greater trochanter
  • Region III - HOs are strictly above the tip of the greater trochanter
  • Grade A - Single or multiple HOs are less than 10 mm in maximal extent without contact with the pelvis or the femur
  • Grade B - HOs are greater than 10 mm without contact with the pelvis but with possible contact with the femur; there is no bridging from the femur to the proximal part of the greater trochanter and no evidence of ankylosis
  • Grade C - Ankylosis by means of firm bridging from the femur to the pelvis is present
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Other Imaging Studies

It takes approximately 2 weeks before sufficient mineralization from HO is detectable on plain films. Computed tomography (CT) or bone scanning may detect this condition sooner. In early HO, bone scans may demonstrate abnormalities on the blood-pool phase and reflect the hypervascularity of the lesion.

The hypervascularity may also be noted on angiography performed early in the natural history of the condition. In mature HO, angiography is of limited use because the lesion becomes avascular.

Ultrasonography (US) has also been suggested as a potentially useful modality for quantitative evaluation of HO during rehabilitation after trauma. [10]

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Histologic Findings

Within 1 week of the index trauma, HO commences with a spindle-cell proliferation. Within a few days of the spindle-cell proliferation, peripheral primitive osteoid develops. About 2 weeks after the index trauma, primitive cartilage and woven bone develop. Trabecular bone begins to appear 2-5 weeks after the index trauma. If a biopsy is performed on HO at 6 weeks after the index trauma, specimens reveal immature undifferentiated tissues centrally with mature lamellar bone peripherally, a finding known as the zonal phenomenon.

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